Curing

October 2015

ROAD MAP TRACK 8

PROJECT TITLE Curing

TECHNICAL WRITERPeter C. Taylor, Director National Concrete Pavement Technology Center Iowa State University

EDITOR Sabrina Shields-Cook

SPONSORFederal Highway Administration

MORE INFORMATIONPeter C. Taylor, Director National Concrete Pavement Technology Center Iowa State University ptaylor@iastate.edu 515-294-5798

“Moving Advancements into Practice”

Best practices and promising technologies that can be used now to enhance concrete paving

The Long-Term Plan for Concrete Pavement Research and Technology (CP Road Map) is a national research plan developed and jointly implemented by the concrete pavement stakeholder community. Publications and other support services are provided by the Operations Support Group and funded by TPF-5(286).

Moving Advancements into Practice (MAP) Briefs describe innovative research and promising technologies that can be used now to enhance concrete paving practices. The October 2015 MAP Brief provides information relevant to Track 8 of the CP Road Map: Concrete Pavement Construction, Reconstruction, and Overlays.

This MAP Brief is available at www.cproadmap.org/publications/MAPbriefOctober2015.pdf.

MAP Brief October 2015

www.cproadmap.org DefinitionCuring is formally defined as “action tak-en to maintain moisture and temperature conditions in a freshly placed cementi-tious mixture to allow hydraulic cement hydration and (if applicable) pozzolanic reactions to occur so that the potential properties of the mixture may develop” (ACI 2013).

In other words, curing involves keep-ing the concrete wet enough and warm enough for hydration to proceed.

Why curing is importantConcrete experts spend a lot of time talk-ing about curing, but in practice it may be an afterthought in the concrete paving process. We also often get away with it, but when disputes occur, the assessment of adequate curing is subjective. The aim of this document is to describe why cur-ing is desirable, particularly for slabs on grade, and to provide recommendations on how it may be achieved.

Hydration of portland cement based mix-tures is a chemical process that requires the presence of cementitious reagents and water to proceed. Allowing a mixture to dry prematurely stops the reaction and the cementi-tious materials effectively become aggregate par-ticles. This is of greater importance when mixtures contain supplementary cementitious materials that generally hydrate slower than plain cement.

Sufficient hydration is critical to ensuring that the pavement or struc-ture achieves the desired

performance goals. Hydration is initially very rapid, but slows over time as shown in figure 1. The longer curing promotes hydration, the greater the performance of the system.

Properties that can be affected by curing include the following:

• Permeabilityisapropertythatinfluencestheabilityofasystemtoresisttheenvi-ronmenttowhichitisexposed.Decreas-ingpermeabilitywillalmostalwaysextendthelifeofasystem.Permeabilityismostcriticalatthesurfacewheretheconcreteisimpactedbytheenvironment,anditisthissamezonethatismoststronglyaf-fectedbythequalityofcuring.

• Strength,whichisaffectedbycuring,isaconcernforstructuralloadingpurposes.However,itshouldbenotedthatexternalcuringislargelyonlyeffectivetoadepthofaboutaninchorlessbelowthesurface.Thismeansthatwhileasmalltestcylinderwillshowreducedstrengthduetopoorcuring,alargecolumnislesslikelytobemeasurablyweakerduetopoorcuring.

• Crackingisincreasinglylikelywithincreasedshrinkagemovements.The

Figure 1. Effect of inadequate curing on concrete properties

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amountofshrinkageisdirectlyrelatedtotheamountofmoisturethatislostduetodryingofthesystemandismostcriticalinslabswithlargesurface-to-volumeratios.Delayingthedryingwillreducewateravailabletoleavethesystembecauseitischemicallyconsumed,anditwillalsoallowthepastetogainincreasedstrength,thusmakingitbetterabletoresistthestressesinducedbytheshrinkage.

In summary, curing is critically important to get the most out of the mixture and to ensure that the system is long lived.

How curing is achievedThe following sections describe best practices for curing.

Temperature managementIn typical practice, curing mostly involves preventing moisture loss; however, when paving under extreme tem-perature conditions (very cold or hot), temperature man-agement becomes a very important aspect of curing. The lower the temperature of a mixture the slower the rate of hydration, with reaction rates being about halved with every 18°F reduction in temperature. While freezing of a mixture is only a concern when pore temperatures are well below 32°F, it is desirable to keep a mixture warm to accelerate development of properties and so reduce the risk of thermal and moisture related cracking.

Thermal gradients across an element can also set up significant stresses and promote cracking. Use of warm water in the mixture, plastic sheeting or blankets will all help to hold heat in the slab and promote hydration. It should be noted that hydration will generate heat within the mixture.

On the other hand, the concrete should not be allowed to get too hot. Temperatures above about 160°F may promote formation of delayed ettringite in some cementi-tious systems, which may cause cracking in the long term. In addition, the higher the temperature in the first stages of hydration, the higher the permeability of the mixture. Use of chilled mixing water and embedded cooling pipes may be needed in large concrete elements. Steam curing of precast systems should be controlled to below about 160°F in the concrete.

Keep it wetInitial curing is a term applied to activities to prevent moisture loss before the concrete sets and finishing activities are completed. Such efforts may be needed in hot, dry, or windy environments. The aim is to ensure that the evaporation rate is lower than the bleeding rate

at the surface of the slab. Activities may include fogging or application of evaporation retarders, primarily to prevent plastic shrinkage cracking. Evaporation retarders applied at controlled rates can be worked back into the surface, but are often abused as finishing aids, which should not be permitted.

Final curing takes places after finishing is complete with the aim of reducing or preventing moisture loss from the concrete for several days. Past or little-used curing meth-ods include the following:

• Floodingaslab(figure2)wasanapproachusedinthe1920sand1930s,butisnolongerpracticalwithcurrentschedules.

• Coveringtheslabwithplasticsheeting(figure3)isnotun-commoninsmallerapplicationslikesidewalks.Onecaveatisthatabuffcolormayformunderthetentedfoldsoftheplasticsheetingunderhotconditions.Placingtheplasticonthesurfacetooearlymayalsodisruptthetexture.

• Tenting(figure4)maybeconsideredifwindsarehigh,butthisisalaborintensiveactivity.

Liquidcuringcompoundsarethemostcommonformofprotectionusedforconcretepavementsandarediscussedinmoredetailbelow.

Figure 3. Colors formed under plastic sheets in hot weather

Figure 2. Flooding of an Iowa roadway (photo courtesy of Mitchell County, Iowa)

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Curing compoundsCuring (or liquid membrane-forming) compounds are typi-cally based on waxes and resins emulsified in water or sol-vent. Emulsified linseed oil cure/sealer compounds are also used for curing some concrete pavements. Pressures to re-duce volatile organic compound (VOC) emissions are forc-ing manufacturers toward water based systems that may be less effective. However, newer products based on poly-alpha-methyl-styrene (PAMS) are proving to be particularly effective. PAMS is a white pigmented curing compound that significantly reduces water loss due to evaporation. PAMS is currently allowed by at least nine state transporta-tion agencies. The Minnesota DOT has been using PAMS-based curing compounds successfully for 15 years. Curing compounds should comply with ASTM C 309, but it should be noted that the precision of this test is low.

Many agencies specify a minimum application rate, typical-ly 15-20 sq yd/gal, but the following should be considered when selecting such limits:

• Increasingroughnessortextureoftheconcretesurfacewillsignificantlyincreasetheamountofproductrequiredtoachieveeffectivecoverageofthewholesurface.

• Applicationratesshouldbebasedonthesolidscontentofproducts,whicharelikelytovarysignificantlybetweenproducts.

• Allowancemustbemadeforwindlosses,andwindshieldsshouldbeprovided.

• Likepainting,experienceandresearchhasshownthattwothincoatsaremoreeffectivethanonethickcoat.

• Nozzlespacingandheightshouldensureuniformandsuf-ficientapplicationrates.

Requirements for application of the curing compound include the following:

Figure 4. A rolling tent structure

•Thesurfacemustbemoist.Waitinguntilsunsetonahotsummerdaywillensurethatthecompoundprovideslittletonobenefit.

• Bleedingmusthaveended,otherwiseawater-richlayermayformbelowthesurfaceacceleratingscaling.Bleedingcontinuesuntilthecementpastehasstiffenedsufficientlytoresistthesettlementofthesolidparticles(typicallylessthan30minutes).

• Applyingcuringcompoundtoasurfacethatistoowetpreventstheproductfromformingasealantlayer.

• Applicationmustbeuniformandconductedbymachine.Figure5showsaphotoofanunacceptablefinish.

• Someagenciesrequirethatthesurfaceshouldbecom-paredasheetofwhitepaperforuniformity.Therearenostandardapproachestomeasuringorensuringadequateapplicationatpresent.Figure6showsaproperlycuredpavement.

• Trafficshouldbekeptofftreatedsurfacesforthedesiredcuringperiodinordertopreventcompoundfrombeingscrubbedoff.

• Allexposedfacesoftheslabshouldhavecuringcompoundappliedtothem.

Figure 5. Inadequate curing compound application

Figure 6. Proper curing compound application

Neither CP Road Map participants or sponsors nor the Federal Highway Administration assumes liability for the information contained in this publication or endorses products or manufacturers mentioned herein.

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Ideally, curing should be provided for 3–7 days, which is about the degree of protection provided by curing com-pounds. Therefore, when used as recommended, no extra treatment is required.

Internal curingInternal curing is a relatively recently adopted approach to providing curing water throughout the depth of a section. This approach was initially aimed at high-performance mixtures that were prone to desiccation, but the literature is indicating that normal bridge and paving mixtures will also benefit.

About 20%–30% of the fine aggregate is replaced with a lightweight fine material (LWFA) that has been wetted for some time before batching. An alternative is to use super absorbent polymers that perform in a similar way. The ex-tra water does not affect w/cm, but can be desorbed from the LWFA or SAP when relative humidity in the paste pores drops below about 95%, and thus promote contin-ued hydration of the system.

The benefit, when compared to surface curing, is that the water is evenly distributed throughout the element, thus reducing differential moisture profiles (and reducing warping). Reported benefits include less shrinkage, better SCM hydration, and improved permeability.

Several states are using the technique as normal practice in bridge decks—pavement applications are more limited at

this time. Additional effort is required to store the LWFA and to control its moisture state before batching.

SummaryCuring is a relatively low cost activity that provides ben-efits well beyond the effort. The fundamentals are sim-ple—keep it wet and keep it warm. The details, however, need to be worked out for each project depending on the weather, materials, and construction constraints.

BibliographyACI Concrete Terminology ACI CT-13 2013

Peter Taylor, Curing Concrete, CRC Press, 2014

Julie M. Vandenbossche, A Review of the Curing Com-pounds and Application Techniques Used by the Minneso-ta Department of Transportation for Concrete Pavements, St. Paul, Minnesota, Minnesota Department of Transporta-tion, November 1999

Toy S. Poole, Curing Portland Cement Concrete Pave-ments, FHWA-HRT-05-038, Federal Highway Administra-tion, McLean, VA, August 2006

Anthony Babcock and Peter Taylor, Impacts of Internal Curing on Concrete Properties, Ames, IA, National Con-crete Pavement Technology Center, IHRB Project TR-676, January 2015